Bandpass filter used in the global communication systems, especially the bulk acoustic wave resonator (BAW Resonator) having the operating frequency suitable for above 2 GHz. Please refer to FIGS. 9A and 9B, which show the sectional schematic views of the bulk acoustic wave resonator of conventional technology and demonstrate the schematic diagrams of the vibration of the piezoelectric material in the vertical direction inducing the vibration in the horizontal direction. The structure of the bulk acoustic wave resonator 14 usually provides a piezoelectric material 12 sandwiched by a bottom electrode 11 and a top electrode 13. Applying an electric field between the top electrode 13 and the bottom electrode 11, utilizing the characteristics of the piezoelectric material, to convert electrical energy into mechanical energy for generating the vertical wave resonance effect in vertical direction such that the high frequency impedance in certain frequency range is very small, therefore the purpose of bandpass filtering is achieved while combining several resonators. However the vibration of the piezoelectric material 12 in vertical direction usually accompanies with the vibration in horizontal direction, thereby the lateral wave is induced. FIG. 9A shows that when the piezoelectric material 12 is extending along the vertical direction, the compression of the piezoelectric material 12 along horizontal direction is accompanied. While FIG. 9B shows that when the piezoelectric material 12 is compressed along the vertical direction, the extension of the piezoelectric material 12 along horizontal direction is accompanied. Furthermore, please also refer to FIG. 9C, which shows the sectional schematic view of the bulk acoustic wave resonator of conventional technology and demonstrates the schematic diagram of the electric field applied between the top electrode and the bottom electrode. The electric field applied between the top electrode 13 and the bottom electrode 11 has the horizontal component near the boundary of the contour of the bulk acoustic wave resonator 14, thereby the electrical energy of the piezoelectric material 12 in horizontal direction is converted into mechanical energy to induce the lateral wave in horizontal direction.
Please refer to FIG. 9D, FIG. 9E and FIG. 9F, which show the top views of the bulk acoustic wave resonator of conventional technology and demonstrate the schematic diagrams of three types of resonance states of the lateral wave respectively. The lateral wave caused by the two factors mentioned the above after total reflection within the bulk acoustic wave resonator may have the opportunity to reach the lateral wave resonance effect in horizontal direction. The later wave resonance effect also called spurious mode. FIG. 9D shows that the lateral wave within the contour 15 (a rectangle contour) of the bulk acoustic wave resonator of conventional technology reaches the resonance state in the left and right direction. FIG. 9E shows that the lateral wave within the contour 15 reaches the resonance state in the forward and backward direction. FIG. 9E shows that the lateral wave within the contour 15 reaches another type of resonance state after total reflection. If the frequency of the later wave resonance effect is too close to the frequency of the vertical wave resonance, then the vertical wave resonance effect will be interfered such that the filter characteristic of the bulk acoustic wave resonator 14 is impacted. Please refer to FIG. 9G, which shows the frequency response graph of the bulk acoustic wave resonator of conventional technology. S11 is the ratio of the transmission electromagnetic wave and the incident electromagnetic wave, while S21 is the ratio of the reflection electromagnetic wave and the incident electromagnetic wave. The spurious modes in FIG. 9G are the spurious modes appeared between the lowest point of S11 and the lowest point of S21. These spurious modes will seriously affect the filter characteristic of the bulk acoustic wave resonator 14.
FIG. 9H shows the top view of another bulk acoustic wave resonator of conventional technology and demonstrates the schematic diagram of the lateral wave. The contour 15 of the bulk acoustic wave resonator is a convex quadrilateral. Thereby the required distance for the lateral wave within the contour 15 to reach the lateral wave resonance effect has been lengthened. Hence, the energy of the lateral wave resonance effect will be reduced, or the frequency of the lateral wave resonance will depart from the frequency of the vertical wave resonance. However conventional technology did not disclose the technical solution of a bulk acoustic wave resonator having a contour formed by at least three curved edges.
Accordingly, the present invention has developed a new design which may avoid the above mentioned drawbacks, may significantly enhance the performance of the devices and may take into account economic considerations. Therefore, the present invention then has been invented.